The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), CMOS active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others, that use an array of microlenses. Semiconductor-based displays using microlenses are also being developed.
Use of microlens significantly improves the photosensitivity of the imaging device by collecting light from a large light collecting area and focusing it onto a small photosensitive area of a photosensor. As the size of imager arrays and photosensitive regions of pixels continue to decrease, it becomes increasingly difficult to provide a microlens capable of focusing incident light rays onto the photosensitive regions of the pixel. This problem is due in part to the increased difficulty in constructing a microlens that has the optimal focal characteristics for the increasingly smaller imager device. Microlens shaping during fabrication is important for optimizing the focal point for the microlens. This in turn increases the quantum efficiency for the underlying pixel array. Utilizing a spherical microlens shape is better for focusing incoming light onto a narrow focal point, which allows for the desired decrease in photosensor size. Spherical microlenses, however, suffer from gapping problems which are undesirable (described below).
Microlenses may be formed through either a subtractive or an additive process. In the additive process, a lens material is formed on a substrate and subsequently is formed into a microlens shape.
In conventional additive microlens fabrication, an intermediate material is deposited onto a substrate and formed into a microlens array using a reflow process. Each microlens is formed with a minimum distance, typically no less than 0.3 microns, between adjacent microlenses. Any closer than 0.3 microns may cause two neighboring microlenses to bridge during reflow. In the known process, each microlens is patterned as a single square with gaps around it. During the reflowing of the patterned square microlenses, a gel drop is formed in a partially spherical shape driven by the force equilibrium of surface tension and gravity. The microlenses then harden in this shape. If the gap between two adjacent gel drops is too narrow, the drops may touch and merge, or bridge, into one large drop. The effect of bridging is that it changes the shape of the lenses, which leads to a change in focal length or, more precisely, the energy distribution in the focal range. A change in the energy distribution in the focal range leads to a loss in quantum efficiency of, and enhanced cross-talk between, pixels. The gaps, however, allow unfocused photons through the empty spaces in the microlens array, leading to lower quantum efficiency and increased cross-talk between respective photosensors of adjacent pixels.
It is desirable to form a microlens array having differently shaped microlenses. However, if the known techniques, which use a single reflow step, were used to form such microlenses, the differently shaped microlenses would have different focal characteristics, which would lead to poor focusing for certain photosensors and/or the need to modify the locations, shape, or symmetries of some photosensors.
It is desirable to enhance the amount of light received from the microlenses and focused on the photosensors of an imager. It is also desirable to form a microlens array with varied sized and shaped microlenses, each having a focal length and focal position optimized for the color or wavelength of light it is detecting. It is also desirable to form a microlens array having minimized gapping between the microlenses without causing bridging during the microlens fabrication reflow process.